The present invention relates to an improved sound-attenuating device. More particularly, but not exclusively, the invention relates to an improved sound attenuating device suitable for use in ventilation, air conditioning, exhaust, flue gas and other air or gas transportation ducts. This invention also has particular application in the hospital and food processing industries.
Sound dampening of such gas transportation ducts can be achieved by use of acoustically absorptive material within the duct to absorb acoustic energy. One of the problems associated with the use of acoustically absorptive material in gas transportation ducts is that such material is susceptible to erosion or teasing out by the flow of gas through the duct. Similarly, contamination, clogging or other fouling of the acoustically adsorptive material may occur as a result of air borne particles present in the flow of gas through the duct. Accordingly, in known sound attenuating devices for use in gas transportation ducts, the acoustically absorptive material is typically protected by the combination of an outer perforated facing (usually of metal) and a thin inner protective layer. The inner protective layer can be a pervious fibrous material such as fibreglass cloth. In dusty environments, or where hygiene requires encapsulation of all fibres"" the inner protective layer may comprise an impervious film to either prevent contamination of the absorptive material by gas borne dust or to prevent tearing out of the acoustically absorptive material.
For effective performance of the sound attenuating device where impervious film is used, neither the outer perforated facing nor the acoustic absorption material should be in direct contact with the protective film but rather there should be an air gap on both sides of it. These air gaps allow the film to move freely in response to acoustic forces generated by pressure waves in the gas allowing the sound to be transmitted more effectively into the acoustically absorptive material for increased attenuation. If the impervious layer is restricted from moving in response to acoustic pressure waves, sound is not transferred as efficiently into the absorptive material and the performance of the sound attenuating device is adversely affected. A further problem which may arise in known sound attenuating devices, occurs where a continuous internal air gap is provided between the outer perforated plate and the film. In these circumstances, the flow of gas through the gas transportation duct can result in the impervious film fluttering and generating its own noise. This fluttering is due to flow between the impervious film and the outer perforated facing.
According to known prior art sound attenuating devices, a spacing layer may be provided between the impervious film and the perforated facing. The spacing is included to prevent contact between the impervious film and the outer perforated facing so as to improve performance of the sound attenuating device. Spacing layers include highly porous cellular or honeycomb structured materials with a high thickness to cell size ratio. However, the known prior art suffers the disadvantage that the movement of the impervious film in response to acoustic forces is restricted, and so the transfer of sound into the absorptive material and sound attenuation is diminished.
In other applications, the prior art includes the use of concrete reinforcing weld mesh as a spacing layer between the impervious film and the outer perforated facing. With this method, the overlapping transverse bars of the mesh allow a continuous gas path between the impervious film and the outer perforated material thereby permitting internal flow along the length of the sound attenuating device and the consequent generation of noise by the flow passing between the impervious film and the transverse bars and between the transverse bars and the perforated facing.
It is accordingly an object of the present invention to provide in one embodiment a sound attenuating device for use in carrying air or gas laden with dust or other particulate matter and thereby provide a more efficient transmission of acoustic energy into an acoustically absorptive material and increased attenuation.
It is a further object of the present invention to provide in one embodiment a sound attenuating device which alleviates or at least minimises the risk of release of fibrous materials used for acoustic absorption which could pose a health risk or be a source of contamination in hospitals or during food preparation or processing.
It is a yet further object of the present invention to provide in one embodiment a sound attenuating device for use in gas transportation ducts which prevents or at least minimises the continuous internal air flow within the sound attenuating device thereby reducing noise generated by means of the flow interacting with the sound absorbing material, the impervious film, the spacer(s) and the outer perforated facing.
Through experimental testing, the applicant has found that the inclusion of regularly spaced transverse spacers between the perforated facing and the impervious film and between the sound absorbing material and the impervious film positioned at right angles to the flow of gas within a gas transportation duct, serve to substantially block internal longitudinal flow between the film and the perforated facing and between the film and the sound absorbing material thereby reducing the generation of acoustic energy caused by the flow of gas between the impervious film and the perforated facing and between the impervious film and the sound absorbing material.
The applicant has further found that increased sound attenuation is achieved if similar air gaps are provided adjacent to both sides of the impervious film to allow unrestricted movement of the film in the void between the perforated facing and the acoustically absorptive material in response to acoustic forces.
The present invention accordingly provides, in one embodiment, a sound attenuating device for use in a gas transportation duct, said device comprising a core of acoustically absorptive material substantially enclosed within an impervious film to prevent direct contact between the acoustically absorptive material and gas flowing through said gas transportation duct, said enclosed absorptive material being maintained in position between at least two perforated facing plates wherein there is provided between the perforated facing and the impervious film, spacing means to prevent contact between the film and the perforated facing and blocking means to impede the internal flow of air between the film and the perforated facing and wherein there is further provided between the impervious film and the acoustically absorptive material, spacing means to prevent contact between the film and the acoustically absorptive material and blocking means to impede the internal flow of gas between the film and the acoustically absorptive material and as a complete assembly to allow unrestricted movement of the impervious membrane in the void between the acoustically absorptive material and the perforated facing.
In a second embodiment of the present invention there is provided a sound attenuating device integrally formed with an internal surface of a gas transportation duct, said device comprising a core of acoustically absorptive material positioned between at least one internal surface of the duct and one or more perforated facing plates and said acoustically absorptive material being covered by an impervious film to prevent direct contact between the acoustically absorptive material and gas flowing through said gas transportation duct, and wherein there is provided between the perforated facing and the impervious film, spacing means to prevent contact between the film and the perforated facing and blocking means to impede the internal flow of air between the film and the perforated facing and wherein there is further provided between the impervious film and the acoustically absorptive material, spacing means to prevent contact between the film and the acoustically absorptive material and blocking means to impede the internal flow of gas between the film and the acoustically absorptive material and as a complete assembly to allow unrestricted movement of the impervious membrane in the void between the acoustically absorptive material and the perforated facing.
In one preferred aspect of the present invention the acoustically absorptive material comprises fibrous material which is totally encapsulated within an impervious layer to ensure there is no release of fibrous material which could pose a health risk or be a source of contamination during food preparation or processing.